The digital GSM system (Global System for Mobile Communication) utilizes TDMA (Time Division Multiple Access). In this technique, each carrier frequency is divided into eight time slots, therewith enabling eight calls to be served simultaneously on one and the same carrier frequency. Each terminal includes a power amplifier in the terminal transmitter part that feeds radio frequency modulated information to an antenna. The function of the power amplifier is to amplify the signals sufficiently for their reception in the nearest base station to be acceptable. This function shall be carried out with the smallest possible power addition from the terminal batteries, because of their limited capacity.
Power amplifiers tend to cause phase distortion in a delivered output signal. This distortion is dependent on output power and will increase with increased output power. This distortion can be expressed in a mathematical vector model: y(t)=re.sup.jwt+f(r). The amplification r, which in this case is the same as the amplitude, is included as a variable in the phase function f(r). The amplification/amplitude can thus be said to have a phase modulating effect on the output signal.
Some non-linear amplifiers exhibit pronounced phase distortion at high powers, although these amplifiers can nevertheless be used in some applications, since they have a greater efficiency than linear amplifiers.
Pulsed amplifiers are used in TDMA applications. The power is thus ramped up to an output power suitable for transmission, in accordance with a ramp function. When transmission is terminated, the power is ramped down in a corresponding manner, in accordance with a ramp function. Upramping and downramping of the output power takes place during very short time intervals. The phase modulation dependent on this upramping and downramping of the output power results in broadening of the frequency spectrum of the output signal, Phase modulation compensation enhances the possibilities of fulfilling given standard requirements (e.g. GSM).
It is known from published PCT Application WO-A1-95/23453 (Motorola) to counteract phase distortion with a feedback that is connected to the power amplifier output and encloses the power amplifier in a phase-locking loop. The power amplifier is fed with a phase modulated signal from a phase modulation control loop that includes a phase-locking loop with a feedback loop connected to the input of the power amplifier. The inclusion of a switching circuit in the phase modulation control loop enables a switch to be made between the two feedbacks. However, it is impossible in practice to achieve fast phase-locking to the correct phase by switching between the feedback loops in the known manner. Because respective upramping and downramping takes place very quickly, problems occur, particularly in the case of TDMA radio applications that use pulsed amplifiers. The overshoots that are generated in the envelope of the output signal are fed back and added to the transients caused by switching between the two feedbacks. Phase-locking therewith takes an unacceptably long time to achieve. Phase-locking may even fail to take place. These drawbacks and problems may result in the total or partial loss of important information stored in a signal. It can therefore be considered desirable to find a novel solution to these drawbacks and problems with which earlier known techniques are encumbered.